Parallel validation error thrown in post validate hook if re-validating

[m-weeks]

Do you want to request a feature or report a bug?
bug

What is the current behavior?
I just recently upgraded from 5.8.4 to 5.9, and the new Parellel validation error that was added in 5.8.5 is causing issues with some functionality I have.

When calling this.validate in a post validate hook, a Parallel validation error will be thrown. My guess is because this.$__.validating must still be set while the hook is running.

I'm not sure if this would be the desired behavior, what do you think? Perhaps the way I am writing the functionality would be better done another way, but I hadn't really thought of one.

If the current behavior is a bug, please provide the steps to reproduce.

Given a schema:

var EmployeeSchema = new Schema({
  name: {
    type: String,
    required: true,
  },
  employeeNumber: {
    type: String,
    validate: {
      validator: function() { return v.length > 5; },
      message: function(props) {
        return `${props.path} must be 5 characters long'`;
       }
    }
  }
});

For theoretical purposes similar to mine, let's say employeeNumber must be retrieved from an external API. This is how I have done it:

EmployeeSchema.post('validate', async function() {
  if (!this.employeeNumber && !this._employeeNumberRetrieved) {
    this.employeeNumber = await getEmployeeNumberFromApi(this.name);
    this._employeeNumberRetrieved = true;
    await this.validate();
  }
});

In my case, it isn't ideal to fetch this data if the other fields are invalid, because it's just going to get thrown out anyway, which is why I am doing it in a post validate hook. But I need to handle the case where if something about the returned employee number is invalid, which is why validation would need to be repeated.

What is the expected behavior?

A document should be able to be re-validated in a post('validate') hook

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Node 10.16.0
Mongoose 5.9.0
Mongodb 4.0.6

[vkarpov15]

Fix will be in v5.9.2. No reason that we can't allow calling validate() in post('validate') middleware since by the time post validate middleware runs, we've already copied over validation errors so you can use Document#invalidate() again.